1. Field of the Invention
The present invention relates to a method of producing micromechanical sensors for the AFM/STM profilometry, which consist of a cantilever beam with a tip at its end and a mounting block at the opposite end. The invention also relates to a sensor head made in accordance with the method of the invention.
2. Description of the Prior Art
The scanning tunneling microscope (hereafter abbreviated STM) has stimulated the development of new techniques for microcharacterization of materials which are based on the use of a very fine tip. One of these techniques involves the atomic force microscope (hereafter abbreviated AFM) which has recently demonstrated the capability to profile and image conductors and insulators.
In the initial design of the AFM which was described by Binnig et al. "Atomic Force Microscope", Phys. Rev. Lett. 56, 1986, pp 930-933 and in European patent document EP-A-0 223 918, a sensor consisting of a spring-like cantilever which is rigidly mounted at one end and carries at its free end a dielectric tip profiles the surface of an object. The force between the object's surface and the tip deflects the cantilever, and this deflection can be accurately measured, for example by a second tip which is part of an STM. A lateral spatial resolution of 3 nm has initially been achieved.
Another version of the AFM includes optical detection instead of an STM detection. In this version a tungsten tip at the end of a wire is mounted on a piezoelectric transducer. The transducer vibrates the tip at the resonance frequency of the wire which acts as a cantilever, and a laser heterodyne interferometer accurately measures the amplitude of the a. c. vibration. The gradient of the force between the tip and sample modifies the compliance of the lever, hence inducing a change in vibration amplitude due to the shift of the lever resonance. Knowing the lever characteristics, one can measure the vibration amplitude as a function of the tip-sample spacing in order to deduce the gradient of the force, and thus, the force itself (Duerig UT, Gimzewski JK, Pohl DW (1986) Experimental Observation of Forces Acting During Scanning Tunneling Microscopy, Phys. Rev. Lett. 57, 2403-2406; and Martin Y, Williams CC Wickramasinghe HK (1987) Atomic Force Microscope-Force Mapping and Profiling on a sub 100-A Scale, J. Appl. Phys. 61(10), 4723-4729).
A most critical component in the AFM is the spring-like cantilever. As a maximum deflection for a given force is needed a cantilever is required which is as soft as possible. At the same time a stiff cantilever with a high eigenfrequency is necessary in order to minimize the sensitivity to vibrational noise from the building. Usually, ambient vibrations, mainly building vibrations, are of the order of &lt;100 Hertz. If the cantilever is chosen such that it has an eigenfrequency f.sub.o .gtoreq.10 kHz, the ambient vibrations will be attenuated to a negligible value. These requirements can only be met by reducing the geometrical dimensions of the cantilever as reflected by the following two equations:
The eigenfrequency fo of the cantilever is given by ##EQU1## wherein E is Young's modulus of elasticity, .rho. is the density, and K is a correction factor close to unity, l is the length, and t is the thickness of the cantilever.
The spring constant of the cantilever on which its sensitivity depends is given by ##EQU2## wherein F is the force which causes the deflection y of the cantilever, E is Young's modulus of elasticity, w is the width, l is the length, and t is the thickness of the cantilever. In accordance with the spring constant term the sensitivity of the cantilever is dependent on its dimensions and on the material of which it consists, with the highest sensitivity being obtained for long, thin and narrow cantilever beams. The width of the cantilever beam should be sufficiently large so that lateral vibrations are suppressed. Also, the width of the beam should permit the fabrication of additional structures, such as tips, thereon. Therefore, a minimum width w of around 10 .mu.m seems reasonable. In practice, C has to be about .gtoreq.1 N/m in order to avoid instabilities during sensing of attractive forces, to prevent excessive thermal vibrations of the cantilever beam, and to obtain a measurable response.
Dimensions of a cantilever beam compatible with C=1 N/m, and f.sub.o =10 kHz are for example: 1=800 .sup..mu. m, w=75 .sup..mu. m and t=5.5 .sup..mu.m.
In the normal deflection mode of the cantilever beam forces in the order of 10.sup.-12 N can be detected. The sensitivity of the sensor head can be further enhanced by vibrating the object to be profiled at the resonance frequency fo of the cantilever beam, as described by G. Binnig et al. in Phys. Rev. Lett. 56 (1986), pp. 930-933.
In the AFM realized in accordance with the afore-mentioned Binnig et al article and with EP-A-0 223 918 the requirements for cantilever and tip were met by a gold foil of about 25 .mu.m thickness, 800 .mu.m length, and 250 .mu.m width to which a diamond fragment was attached with a small amount of glue. Another proposal used microfabrication techniques to construct thin-film (1.5 .mu.m thick) SiO2 microcantilevers with very low mass on which miniature cones could be grown by evaporation of material through a very small hole as described by Albrecht et al. "Atomic Resolution with the Atomic Force Microscope on Conductors and Non-conductors", J. Vac. Sci. Technol., 1988, pp. 271-274.
From the foregoing description of the state of the art it was known to construct, in a first process step, cantilevers, and, in a second process step, to attach tips thereto. It will be obvious to those skilled in the art that the construction of a cantilever with tip of that type is extremely delicate and prone to low yield.
The following publications relating to micromechanics are noted as examples of the prior art::
Petersen, KE, Dynamic Micromechanics on Silicon: Techniques and Devices, Vol. ED-25, No. 10, October 1978, pp. 1241-1250;
Petersen, KE, Silicon as a Mechanical Material, Proc. of the IEEE, Vol. 70, No. 5, May 1982, pp. 420-457; and
Jolly, RD, Muller, RS, Miniature Cantilever Beams Fabricated by Anisotropic Etching of Silicon, J. Electrochem Soc.: Solid-State Science and Technology, December 1980, pp. 2750-2754.